Composite products made from lignocellulosic materials include “composite boards”, which term is used herein to mean oriented strand boards (OSBs), wafer boards, straw boards, fibre boards, chip boards and particle boards wherein the board substrate is prepared by applying an adhesive to lignocellulosic particles, chips or fibres, specifically wood particles, wood chips and lignocellulosic particles, and subsequently forming the lignocellulosic material into the desired board through application of heat and pressure.
Adhesives currently used by manufacturers of various wood composite products include urea-formaldehyde, phenol-formaldehyde, melamine-urea-formaldehyde, melamine-formaldehyde resins (“UF”, “PF”, “MUF”, “MF”), and isocyanate polymers (“MDI”). For example, U.S. Pat. No. 6,297,313 to Hsu teaches a sprayable liquid adhesive system comprising an aldehyde resin and an isocyanate polymer which are combined prior to being sprayed onto lignocellulosic particles.
At a number of OSB and composite board mills, MDI is the predominant or sole binder system. In some layers in the composite board, MDI has been partially replaced by other resin adhesives including phenolic resin (powder or liquid) to achieve specific properties or costs for the board.
These adhesives suffer from a number of disadvantages. First, the ill-effects of formaldehyde on human health are well known. Many of the above adhesives contain free formaldehyde or release formaldehyde during the manufacturing process. In some cases, formaldehyde is released even from composite boards formed using some of the above adhesives over the useful life of the manufactured board. Isocyanate polymers tend to be safer once the composite board has been through the press, but are still not entirely free of health risks prior to and during application as an adhesive binding system. They can react with moisture on the skin or moisture in the lungs if inhaled as atomized isocyanate polymer or isocyanate polymer-coated wood dust. Also, isocyanate polymers can cause manufacturing problems since they can bond to metals (i.e., metal plates and presses) and are more expensive than other conventional adhesives per unit weight basis.
Second, the above aldehyde polymers are derived from non-renewable petroleum based resources via multiple step chemical processes that often release carbon dioxide and other emissions into the environment. It is therefore desirable to limit their use where possible for environmental reasons.
Raw materials used to manufacture many aldehyde polymers include phenol, formaldehyde and caustic soda, all chemicals that are considered dangerous to handle in the workplace.
Third, liquid adhesive systems tend to suffer from short or limited storage and pot life. “Pot life” means the amount of time following the addition of a catalyst in which the liquid adhesive system is useful for the desired application catalyzed. Liquid systems consist of components which, when mixed, begin to react and cure. After a certain period of time, the system may lose its utility, i.e. cannot be used. With catalyzed aldehyde liquid systems, the individual components must be measured very accurately and be present in certain specified amounts in order to provide a solution which will achieve the desired properties upon curing. For these reasons, powdered adhesive systems are preferred since they do not suffer from the same limited storage and pot life problems.
Tannin has been used as an adhesive component for at least 50 years. It is derived from natural renewable resources (i.e. trees and plants). Certain kinds are widely available, are extracted from the bark of trees cut for lumber, and are not known to cause problems to health. However, the use of tannins in adhesives is not widespread, especially for OSB board manufacture, since most liquid aldehyde tannin adhesives systems suffer from pot life constraints.
Normally, the tannin is used in conjunction with another aqueous based adhesive system, with a catalyst being added to assist in the curing. The catalyst becomes an active part of the binder system for the composite wood being glued. The most common practice is to add a formaldehyde ‘donor’ to a solution/dispersion of tannin mixed with an aqueous phenolic or urea formaldehyde resin adhesive. The dry weight of tannin to the adhesive normally totals 20% w/w and the formaldehyde source is normally paraformaldehyde or urea-formaldehyde concentrate (UFC 80% w/w or UFC 85% w/w) added at a weight ratio of 1:10 (catalyst:tannin). Although used commercially, these aqueous solution mixed systems suffer from the short pot life of an advancing reacting pre-polymer mixture, and problems with the amount of excess formaldehyde being added to assist in the cross-linking or curing. Few OSB mills have been able to overcome the problems encountered in preparing an efficient tannin containing OSB binder system because of the continuous nature of the OSB panel manufacturing process and because of the problems associated with increased free formaldehyde emissions resulting from the addition of formaldehyde ‘donor’ catalysts/reactants.
In U.S. Pat. No. 5,407,980 to Pizzi et al., tannin is used as an accelerator to speed up the curing process. This patent teaches a liquid adhesive composition for manufacturing plywood for exterior application including: a) 121 parts by weight phenolic resin: b) 5 to 121 parts by weight isocyanate polymer: c) 1 to 40 parts by weight tannin selected from the group consisting of pine, quebracho, mimosa, and combinations thereof: d) 1 to 15 parts by weight paraformaldehyde or formaldehyde solutions in water: e) 5 to 50 parts by weight of water: and f) an amount of filler comprising inorganic and/or organic materials for providing the composition with the desired viscosity. Pizzi et al. further teaches that the added formaldehyde or paraformaldehyde is necessary to achieve the required hard gel.
Thus, the solution of Pizzi et al. contains added formaldehyde solutions or paraformaldehyde additions. Unreacted formaldehyde escaping during the manufacturing process or from the finished board is a potential health hazard. In addition, like other prior art liquid adhesive compositions, the solution has a limited pot life and the quantity of the various component ingredients such as paraformaldehyde is critical to performance. Due to the rapid advancing polymerization of certain types of tannins (e.g. unhydrolyzed tannins), the Pizzi et al. solution is restricted in terms of the types and amounts of tannin which may be used.
U.S. Pat. No. 5,532,330 also to Pizzi et al. teaches a liquid heat-curable, formaldehyde-free binding agent comprising a polyflavonoid type tannin and, as a curing catalyst, a compound having a weak acid reaction curable at relatively high temperatures and compatible with cellulose containing materials such as wood chips for the production of particle boards. The curing catalyst may be silicon dioxide or boric acid. As the liquid binding agent must be mixed prior to use, it also suffers from the disadvantages of limited pot life and the requirement to precisely measure and mix in the individual components, usually by a batch process up to several times in a day at the board manufacturing plant. Furthermore there is a need to inventory and maintain the quality control of the individual components used to make up the catalyzed adhesive system.
Manufacturers of adhesive systems for composite panels continue to search for supplements to or replacements for adhesives presently used, especially those which are based on urea- and phenol-formaldehyde adhesives and isocyanate polymers because of environmental, health, cost, and performance concerns. There is still a need for new adhesive compositions or systems which (1) can be formulated and used with relative ease while providing a composite board having the desired physical properties; (2) are less harmful to the environment by employing products derived from natural and renewable resources; (3) are less harmful to human health by reducing or eliminating the amount of free formaldehyde released from composite boards, during the lifetime of the finished composite board, and from presses during the manufacturing process; and (4) are relatively cost-effective to manufacture and use. An object of the present invention is to at least partially address these needs.